Official: Gas leak caused Mexico oil company blast

An employee culls through debris searching for office documents amid the rubble left from an office building explosion, in Mexico City, Saturday, Feb. 2, 2013. A Thursday blast collapsed the lower floors of the Mexican state-owned oil company Petroleos Mexicanos, or Pemex, headquarters, crushing at least 33 people beneath tons of rubble and injuring 121. A Pemex spokesman said the floors hit by the explosion housed administrative offices. (AP Photo/Marco Ugarte)
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An employee culls through debris searching for office documents amid the rubble left from an office building explosion, in Mexico City, Saturday, Feb. 2, 2013. A Thursday blast collapsed the lower floors of the Mexican state-owned oil company Petroleos Mexicanos, or Pemex, headquarters, crushing at least 33 people beneath tons of rubble and injuring 121. A Pemex spokesman said the floors hit by the explosion housed administrative offices. (AP Photo/Marco Ugarte)
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Soldiers remove debris after an explosion at the state-owned oil company PEMEX office complex in Mexico City, Sunday Feb. 3, 2013. Mexico's state-owned oil company says rescuers have found another body amid the rubble of a headquarters building damaged by a still-unexplained blast. The find raises the death toll of Thursday's explosion to about 34 people. (AP Photo/Marco Ugarte)— AP

Soldiers remove debris after an explosion at the state-owned oil company PEMEX office complex in Mexico City, Sunday Feb. 3, 2013. Mexico's state-owned oil company says rescuers have found another body amid the rubble of a headquarters building damaged by a still-unexplained blast. The find raises the death toll of Thursday's explosion to about 34 people. (AP Photo/Marco Ugarte)
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MEXICO CITY 
A gas buildup under a building in the headquarters of Mexico's state oil company caused a blast that killed 37 people and wounded dozens, the attorney-general said Monday, ending days of near-total silence by authorities about the petroleum giant's worst disaster in more than a decade.

Attorney-General Jesus Murillo Karam said an investigation by Mexican, Spanish, U.S. and British experts found no evidence of explosives in the blast that collapsed several lower floors of the Petroleos Mexicanos administrative building on Thursday afternoon. He said investigators believed that an electrical spark or other source of heat had detonated the gas.

With the exception of three victims, none of those killed had the burn marks or damaged ear drums that are typical evidence of a bombing, he said. Nor was there any sign of a crater or fracturing of the building's steel beams, also common signs of the detonation of an explosive device.

Murillo said officials had yet to discover the source of what initial evidence indicated to be methane gas that leaked from a duct or tunnel or came from the sewer system and built up in the basement of the building.

Murillo said that an independent contractor had told investigators that he was working with a crew of three men performing maintenance in the basement of building B2. The contractor said the basement wasn't lit, so his crew had rigged illumination by attaching a crude electric cable to a power source in the ceiling.

The contractor told investigators that seconds after he moved to a higher floor, he heard a noise and then the building was rocked by an explosion. The crew of three men was found dead in the lower basement with burn marks, one with a fragment of cable stuck to his body. They had no evidence of the dismemberment typical in the detonation of explosives.

Murillo described the blast as a "diffuse" explosion whose blast moved slowly and horizontally, typical of the detonation of a cloud of gas, rather than an explosion that would have emanated from a relatively compact source like a bomb.

He said laboratory tests had turned up "zero" evidence of any explosive.

"We've been able to determine that the explosion was caused by an accumulation of gas in the basement of the building," he said. "This explosion, at its peak, generated an effect on the structures of the floors of the building, first pushing them up and then causing them to fall, and that was the primary cause of deaths in the building."

The announcement late Monday ended days of a near-total lack of information about the potential cause of the incident. The sparse information spawned a torrent of complaints about government secrecy and speculation about the cause of the blast, most focusing on the possibility that it was intentional.

The suspicions of foul play became so intense that Murillo insisted on displaying photos of a backpack found in the rubble in order to prove to the public that it contained makeup, and not a suspicious, potentially explosive device as reported by some Mexican media earlier in the day.